Fluorocarbon and organosilicon thin films produced by chemical vapor deposition have a wide variety of applications, ranging from biocompatible coatings for medical implants, to low K dielectrics in integrated circuits. Noort, R. V, Black, M. M. Biocompatibility of Clinical Implant Materials; Williams, D. F. Ed.; CRC Press: Boca Raton, 1981; Vol. 2, p 79-98; Chawla, A. S. Artif. Organs 1979, 3, 92; Chawla, A. S. Biomaterials 1981, 2, 83; Ocumpaugh, D. E., Lee, H. L., Biomedical Polymers; Rembaum, A., Shen, M., Eds.; Marcel Dekker: New York, 1971; p 101; Thomson, L. A., Law, F. C., James, K. H., Rushton, N. Biomaterials 1991, 12, 781; Guidoin, R., Chakfe, N., Maurel, S., How, T., Batt, M., Marois, M., Gosselin, C. Biomaterials 1993, 14, 678; and Lau, K. K. S., Gleason, K. K. Mater. Res. Soc. Symp. Proc. 1999, 544, 209; Rosenmayer, T., Huey, W. Mater. Res. Soc. Symp. Proc. 1996, 427, 463; Peters, L. Semicond. Int. 2000, 23, 108; Loboda, M. J. Microelect. Eng. 2000, 50, 15; and Grill, A., Patel, V. J. Appl. Phys. 1999, 85, 3314. Fluorocarbon films have been found to be biocompatible and have low dielectric constants. However, they also have a high degree of roughness and do not adhere well to silicon substrates. Silicone films are biocompatible and offer the additional advantages of excellent adhesion to silicon substrates and superior thermal stability. But the dielectric constant of these films is higher than that of the fluorocarbon films. A fluorocarbon-organosilicon copolymer film therefore has the potential to incorporate the desirable attributes of each class of material into a single film.
The addition of fluorine to organosilicon films is anticipated to lower the dielectric constant, increase electrical resistivity, reduce surface energy, increase hydophobicity, and reduce permeability to water. All of these trends are favorable for biopassivation applications. The copolymer films can also retain the desirable adhesion characteristics and mechanical properties of the organosilicon homopolymer. Flexible copolymer films could also be used as biopassivation coatings on biomedical device components.
Transparent, tough, hard, scratch resistant films having extreme hydrophobicity would make excellent protective and dirt resistant coatings on window glass, windshields, and eyewear. Since the substrate remains at low temperature during the process, temperature sensitive materials such as plastics and fabrics can also be coated. Potential applications include anti-fouling coatings on marine vessels and equipment, coatings for food containers, and biological and chemical laboratory equipment. The hybrid copolymer can also serve as an intermediate transition layer for graded coatings such as the stack substrate-organosilicon-copolymer-fluoropolymer. Such an arrangement can produce an adherent interface with a hydrophobic surface, or even a film in which one surface is hydrophobic and the other hydrophilic.
Organosilicon or fluorocarbon homopolymers can be coated onto surfaces by a number of techniques such as spin-on coating, casting or chemical vapor deposition. An important advantage of chemical vapor deposition (CVD) is the ability to create copolymers that are difficult to synthesize by bulk or solution techniques, such as fluorocarbon-organosilicon copolymers. Fluorocarbon polymers are normally synthesized by free radical polymerization, whereas polysiloxanes are made by ionic polymerization techniques. Synthesis of a copolymer would thus require a transformation from ionic polymerization to free radical polymerization (or vice versa). Although several transformation techniques have been reported in the literature, to our knowledge, none of these methods have been applied to the synthesis of fluorocarbon-organosilicon copolymers. Serhatli, I. E., Galli, G., Yagci, Y., Chiellini, E. Polym. Bull. 1995, 34, 539; Cunliffe, A. V., Hayes, G. F., Richards, D. H. J. Polym. Sci. (B) 1976, 14, 483; Souel, T., Schue, F., Abadie, M., and Richards, D. H. Polymer 1977, 18, 1292.
Among the different CVD techniques available, hot-filament CVD (HFCVD, also known as pyrolytic or hot-wire CVD) is unique in several respects. In HFCVD, a precursor gas is thermally decomposed by a resistively heated filament. The resulting pyrolysis products adsorb onto a substrate maintained at around room temperature and react to form a film. HFCVD does not require the generation of a plasma, thereby avoiding defects in the growing film produced by UV irradiation and ion bombardment. In addition, films produced by HFCVD have a better-defined chemical structure because there are fewer reaction pathways than in the less selective plasma-enhanced CVD method. HFCVD provides films with a substantially lower density of dangling bonds, i.e. unpaired electrons. Further, HFCVD has been shown to produce films that have a low degree of crosslinking. Limb, S. J., Lau, K. K. S., Edell, D. J., Gleason, E. F., Gleason, K. K. Plasmas and Polymers 1999, 4, 21.
HFCVD has been used to deposit fluorocarbon films that are spectroscopically similar to poly(tetrafluoroethylene) (PTFE) as well as organosilicon films that consist of linear and cyclic siloxane repeat units. Limb, S. J., Lau, K. K. S., Edell, D. J., Gleason, E. F., Gleason, K. K. Plasmas and Polymers 1999, 4, 21. Few attempts have been made to create fluorocarbon-organosilicon copolymers by CVD, and these have been limited to plasma-enhanced CVD (PECVD). Sakata, J., Yamamoto, M., Tajima, I. J. Polym. Sci. (A) 1988, 26, 1721; Kim, D. S., Lee, Y. H., Park, N. Appl. Phys. Lett 1996, 69, 2776; Shirafuji, T., Miyazaki, Y., Nakagami, Y., Hayashi, Y., Nishino, S. Jpn. J. Appl. Phys. 1999, 38 Pt. 1 No. 7B, 4520; H. Kotoh, M. Muroyama, M. Sasaki, M. Iwasawa, Jpn. J. Appl. Phys. 1996, 35 Pt. 1, No. 2B, 1464; and P. Favia, G. Caporiccio, R. d'Agostino, J. Polym. Sci. Part A: Polym. Chem. 1994, 32, 121-130. Sakata et al. obtained thin films using hexamethyldisiloxane (HMDSO) and tetrafluoromethane (CF4) by plasma-enhanced CVD. Sakata, J., Yamamoto, M., Tajima, I. J. Polym. Sci. (A) 1988, 26, 1721. The structure of the films was found to be different from a simple blend of fluorocarbon and organosilicon polymers. In other words, the polymer film did not consist of simple block or random copolymers. The authors observed the presence of Si—F bonds, and the data presented indicates that most of the fluorine in the films was bonded to silicon.
Similar results were obtained by Kim et al. with HMDSO and perfluorobenzene (C6F6). Kim, D. S., Lee, Y. H., Park, N. Appl. Phys. Lett 1996, 69, 2776. This investigation also included dielectric constant measurements and adhesion tests. The dielectric constants of the copolymer films were found to lie between those of the respective homopolymeric films, between 2 (pure fluorocarbon) and 4 (pure organosilicon). Annealing the films brought about a slight decrease in the dielectric constant. Adhesion of these films to silicon substrates was measured using the ASTM tape test and was determined to be better than that of pure fluorocarbon films.
Favia et al. investigated the plasma-enhanced CVD of a cyclic fluorinated siloxane, (3,3,3-trifluoropropyl)methylcyclotrisiloxane. P. Favia, G. Caporiccio, R. d'Agostino, J. Polym. Sci. Part A: Polym. Chem. 1994, 32, 121-130. The authors examined the effects of varying substrate temperature and substrate bias on the deposition rate and chemical composition of the films. Films deposited with substrate temperatures below 200° C. were determined to be structurally similar to the precursor. The carbon and hydrogen content of the films was found to decrease at higher substrate temperatures along with the deposition rate. Increasing the substrate bias resulted in greater crosslinking and higher deposition rate. The authors emphasize the absence of Si—F, Si—H and O—H bonds in their films.
There still exists a need for a reliable method of depositing fluorocarbon-organosilicon copolymer thin films with well-resolved bonding environments. We describe herein HFCVD methods for forming fluorocarbon-organosilicon films without the complex structures and undefined spectroscopic features associated with PECVD methods. Extensive spectroscopic characterization confirms the presence of covalent bonds between CF2 groups and siloxane-based polymeric units in the film.